Embodiments of the invention relate generally to diagnostic imaging and, more particularly, to an apparatus for scatter reduction for CT imaging and a method of fabricating same.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotating about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
In recent years CT systems have increasing z-coverage in order to shorten scan times and reduce overall dose. The goal has been to obtain an image of an object, such as a cardiac region, in a single rotation. As CT systems have grown in z-coverage (i.e., increased numbers of slices), however, scatter has become an increasingly significant factor. For example, for a 16-slice scanner with 10 mm z-coverage, the scatter-to-primary ratio (SPR) is less than 10% for a 35 cm poly phantom. When the z-coverage increases to 40 mm (or 64 slices), the SPR increases to 20% for the same size phantom. It is well-known that an increased SPR degrades image quality due to image artifact and noise increase.
Many attempts have been made in the past to improve the scatter performance of CT systems. For example, by increasing the aspect ratio of post-patient collimation plates, the scatter rejection capability can be significantly improved. The aspect ratio for a collimator is typically defined as the collimator plate height (H) divided by the aperture width (W). In general, the higher the aspect ratio, the better is the scatter rejection capability. However, as known in the art, the scatter rejection capability of a one-dimensional (1D) configuration is limited since scattered radiation in the y-z plane can still reach the detector without being blocked.
To overcome the shortcomings of 1D collimation, two-dimensional (2D) collimation may be used in order to improve the scatter rejection capability of the system. In such a configuration, collimator plates are placed orthogonal to each other and all point (or focus) to the x-ray focal spot, in order to block the scattered radiation in this direction. It has been shown that with the additional collimation, the SPR can be reduced to less than 10% for a 16 cm z-coverage system using a 35 cm poly phantom. The drawback of this approach, however, is a significant increase in the cost to the design and manufacturing.
Therefore, it would be desirable to design an apparatus and method of fabricating a collimator to reduce an amount of scatter and reduce overall cost of an imaging system.